Is it possible to export .MDL files?
There is a game development framework called Urho which utilizes 3D models as .mdl files for the model and .png for the texture, you don't need to download the whole framework but they have a plugin for blender that lets you export a model in those formats.

See http://urho3d.wikia.com/wiki/Blender_to_Urho3D_Guide

Simple Installation: https://github.com/reattiva/Urho3D-Blender

^^^ go to that page and  click the green button where it says 'clone or download' then click Download Zip

Once you have the zip file, open Blender, go to File -> User Preferences -> Addons.

At the bottom click  Install From File... and give it the zip file.

Then click the checkbox to enable, and to see the export option:

The addon is located in the "Properties" panel, at the end of the "Render" page (camera icon)

Note: In Urho, the textures are mapped to the models via xml material files, this is due to the Xamarin (mobile .NET) architecture of Urho projects. If all you need however, is the .mdl then this is the solution for you :)